Identifying the location of a wireless telephone making an emergency call is a matter of great importance and intense interest. Unlike a landline telephone whose telephone number is associated with a particular address, wireless telephones can be used from anywhere and knowledge of the telephone number of the wireless telephone making the call contributes nothing toward the knowledge of the location of the wireless telephone at the time of the call.
Global positioning system (GPS) receivers have great value in determining the location of a user, but the use of a GPS receiver is not a reliable solution for determining the location of a user inside a building, especially in a critical application such as providing a user's location to an emergency call system. It is possible for a receiver with a good view of the sky to determine its position. Such a receiver can be integrated into a wireless telephone and a wireless telephone from which an emergency call is made can transmit location information provided by the GPS receiver to the central exchange, which in turn can relay this information to emergency personnel. However, even in outdoor use, GPS has drawbacks that detract from its desirability as the sole source of location information in critical situations. A GPS receiver may take several minutes to achieve a location fix, particularly if the GPS receiver has been transported more than a few hundred miles since last achieving a location fix. Moreover, if obstructions prevent the receiver from achieving a clear line of sight to a sufficient number of satellites, the receiver may fail to achieve a fix.
The use of a GPS receiver within a building introduces even more difficulties. Most building materials obstruct the receiver's view of satellites, preventing the receiver from achieving a location fix. In addition, the accurate determination of a location within a building requires altitude information as well as latitude and longitude information, and the altitude information provided by a GPS is much less accurate than the latitude and longitude information provided. GPS should not be relied on to determine location in a building in critical applications such as location identification of a telephone making an emergency call.
There exists, therefore, a need for location identification systems and methods that will operate within a building.